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The oceanic uptake of anthropogenic CO2 is primarily a physical response to rising atmospheric CO2 concentrations. Whenever the partial pressure of a gas is higher in the atmosphere over a body of water, the gas will diffuse into that water until the partial pressures across the air–water interface are equilibrated. There is no evidence that the rising atmospheric CO2 concentrations have had a measurable impact on biological processes in the ocean. The growth rate of the primary producers in the ocean (phytoplankton) is generally limited by either light or nutrient availability, not carbon. It is possible that climate changes (e.g., ocean temperature or circulation changes) may be affecting the ocean carbon system, but these effects are thought to be small for the 19th and 20th centuries.
Several independent approaches have been used to estimate the modern oceanic uptake rate of anthropogenic CO2. Table 3.3 shows a summary of the ocean observations and model estimates of the anthropogenic CO2 uptake in the 1990s (in Pg C/year). Most of the models are in reasonably good agreement with the flux estimates from the observational data when corrected for the pre-industrial carbon flux.
Models suggest that anthropogenic CO2 uptake occurs everywhere in the surface ocean, even in areas that have a total net CO2 flux out of the ocean. For example, the pCO2 of sea water in the eastern equatorial Pacific is much higher than atmospheric pCO2. This sea–air difference in pCO2 causes the ocean to release CO2 to the atmosphere. As atmospheric CO2 concentrations rise, the difference between the sea water pCO2 and atmospheric pCO2 is decreased and the rate of CO2 loss drops. The additional CO2 that is stored in the sea water, which would have been lost to an atmosphere with lower pCO2 values, is referred to as the anthropogenic uptake.
It is important to note that there is a difference between CO2 uptake and CO2 storage. While there may be a large anthropogenic CO2 uptake in the equatorial Pacific, the water is quickly transported off the equator and the anthropogenic CO2 is actually stored in the subtropical gyres (Gloor et al., 2003). Ocean carbon models suggest that the high-latitude Southern Ocean is also a region with large anthropogenic CO2 uptake, but the anthropogenic CO2 is stored further to the north where mode and intermediate water masses are formed. Water mass formation regions are areas in which water is moved from the surface into the ocean interior. Once the waters leave the surface, the anthropogenic CO2 is isolated from the atmosphere and stored until these waters return to the surface.
Recognizing the need to constrain the oceanic uptake, transport and storage of anthropogenic CO2 during the anthropocene as well as to provide a baseline for future estimates of oceanic CO2 uptake, two international ocean research programmes, the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS), jointly conducted a comprehensive survey of inorganic carbon distributions in the global ocean in the 1990s (Wallace, 2001). After completion of the US field programme in 1998, a 5-year effort was started to compile and rigorously quality control the US and international data‑sets including a few pre-WOCE data-sets in regions that had limited data (Key et al., 2004). The final data-set, with 9618 hydrographic stations collected on 95 cruises, provides the most accurate and comprehensive view of the global ocean inorganic carbon distribution available (see http://cdiac.esd.ornl.gov/oceans/glodap/Glodap_home.htm). By combining these data with a back calculation technique (Gruber et al., 1996) for isolating the anthropogenic component of the measured DIC, Sabine et al. (2004b) estimated that 118 ± 19 Pg C has accumulated in the ocean between 1800 and 1994. This inventory accounts for 48% of the fossil fuel and cement manufacturing CO2 emissions to the atmosphere over this time frame.
A map of the anthropogenic CO2 ocean column inventory (Fig. 3.5) shows that CO2 is not evenly distributed in space. More than 23% of the inventory can be found in the North Atlantic, a region covering ~15% of the global ocean. By contrast, the region south of 50°S represents approximately the same ocean area but has only ~9% of the global inventory (Sabine et al., 2004b). Despite the relatively slow equilibration rate for CO2 in sea water (~1 year versus weeks for oxygen), uptake at the surface does not fully explain the spatial differences in storage. The primary reason for these differences is due to the slow mixing time in the ocean interior and the fact that waters move into the deep ocean only in a few locations. The highest inventories are found in locations where mode and intermediate waters move anthropogenic CO2 into the ocean interior (e.g., the northern North Atlantic or in the southern hemisphere associated with the subtropical convergence zone at 40°S–50°S; Fig. 3.5).
Fig. 3.5. Global map of anthropogenic CO2 column inventory in mol/m2. (From Sabine et al., 2004b.)
One exception to the observation of higher inventories associated with water mass formation regions is the fact that no large inventories are associated with the formation of bottom waters around Antarctica. There are several possible reasons:
In reality, it is likely to be a combination of all these factors that has limited our ability to detect substantial anthropogenic CO2 concentrations in the bottom waters around Antarctica.
Figure 3.6 shows sections of anthropogenic CO2 in the Atlantic, Pacific and Indian oceans. One feature that clearly stands out in these examples is that most of the deep ocean has still not been exposed to elevated CO2 levels. Nearly 50% of all the anthropogenic CO2 is stored in the upper 10% of the global ocean (depths less than 400 m) and detectable concentrations of anthropogenic CO2 average only as deep as 1000 m. The global ocean is far from being saturated with CO2. This further illustrates that the primary rate-limiting step for oceanic carbon uptake is not the exchange across the sea–air interface, but the rate at which that carbon is transported into the ocean interior. Model studies suggest that the ocean ultimately will absorb 70–85% of the CO2 released from human activity, but given the slow mixing time of the ocean, this will take millennia to accomplish (Le Quéré and Metzl, 2004).
Fig. 3.6. Representative sections of anthropogenic CO2 (µmol/kg) from the (a) Atlantic, (b) Pacific and (c) Indian oceans. Insets show maps of the station locations used to generate the sections.
By combining the Sabine et al. (2004b) estimate of the anthropogenic CO2 that has accumulated in the ocean between 1800 and 1994 with a synthesis of the average uptake estimate for the last 20 years (Sabine et al., 2004a), we can evaluate potential changes in the decadal-scale uptake rate of anthropogenic CO2 by the ocean. Table 3.4 shows the change in carbon inventories during the first 180 years of the anthropocene versus inventory changes over the last 20 years. These estimates suggest that the oceanic uptake of net CO2 emissions decreased from ~44% during the first period to ~36% over the last two decades. Although this difference is not statistically significant, there is a suggestion that the oceanic uptake efficiency is decreasing with time.
Several countries have initiated programmes to evaluate decadal-scale changes in oceanic CO2 uptake. For example, Ocean Station Papa in the north-eastern Pacific Ocean has been sampled for oceanic carbon on a semi-regular basis for the last 40 years (Signorini et al., 2001). Within the USA, the Hawaii Ocean Time-series (HOT) programme and Bermuda Atlantic Time-series Study (BATS) have been measuring carbon concentrations in the water column for more than 15 years. These projects have focused most of their attention on seasonal to interannual variability, but are beginning to have records long enough to see longer-term variability in CO2 uptake (e.g., Bates, 2001; Gruber et al., 2002; Dore et al., 2003; Keeling et al., 2004). Additional sites are also being examined by European and Asian countries.
Changes in the carbon concentrations along hydrographic sections sampled several years apart can also provide useful information on decadal-scale CO2 uptake. At least seven countries have agreed to coordinate hydrographic survey cruises to monitor the decadal-scale changes in ocean carbon inventory. For example, the USA CLIVAR/ CO2 Repeat Hydrography Program has outlined 19 cruises that will reoccupy sections that were last sampled in the 1990s. To date, six lines have been run. Preliminary results have suggested interesting basin-to-basin differences in the inferred uptake rates on these lines (e.g., 0.7 mol/m2/year in the North Atlantic versus 1.1 mol/m2/year in the North Pacific; see Feely et al., 2005; Wanninkhof et al., 2005). These changes in oceanic uptake may reflect changes in ocean circulation and/or the enactment of feedback mechanisms in the ocean that can serve to either enhance or reduce the uptake of anthropogenic CO2 in the ocean.
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